A variety of implantable medical devices for delivering a therapy and/or monitoring a physiological condition have been clinically implanted or proposed for clinical implantation in patients. Some implantable medical devices may employ one or more elongated electrical leads carrying stimulation electrodes, sense electrodes, and/or other sensors. Implantable medical devices may deliver electrical stimulation or fluid therapy and/or monitor conditions associated with the heart, muscle, nerve, brain, stomach or other organs or tissue. Implantable medical leads may be configured to allow electrodes or other sensors to be positioned at desired locations for delivery of stimulation or sensing. For example, electrodes or sensors may be carried at a distal portion of a lead. A proximal portion of the lead may be coupled to an implantable medical device housing, which may contain circuitry such as stimulation generation and/or sensing circuitry.
Implantable medical devices, such as cardiac pacemakers or implantable cardioverter-defibrillators, for example, provide therapeutic electrical stimulation to the heart via electrodes carried by one or more implantable leads. The electrical stimulation may include signals such as pulses or shocks for pacing, cardioversion or defibrillation. In some cases, an implantable medical device may sense intrinsic depolarizations of the heart, and control delivery of stimulation signals to the heart based on the sensed depolarizations. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate electrical stimulation signal or signals may be delivered to restore or maintain a more normal rhythm. For example, in some cases, an implantable medical device may deliver pacing pulses to the heart of the patient upon detecting tachycardia or bradycardia, and deliver cardioversion or defibrillation shocks to the heart upon detecting fibrillation.
Leads associated with an implantable medical device typically include a lead body containing one or more elongated electrical conductors that extend through the lead body from a connector assembly provided at a proximal lead end to one or more electrodes located at the distal lead end or elsewhere along the length of the lead body. The conductors connect stimulation and/or sensing circuitry within an associated implantable medical device housing to respective electrodes or sensors. Some electrodes may be used for both stimulation and sensing. Each electrical conductor is typically electrically isolated from other electrical conductors and is encased within an outer sheath that electrically insulates the lead conductors from body tissue and fluids.
Cardiac lead bodies tend to be continuously flexed by the beating of the heart. Other stresses may be applied to the lead body during implantation or lead repositioning. Patient movement can cause the route traversed by the lead body to be constricted or otherwise altered, causing stresses on the lead body. The electrical connection between implantable medical device connector elements and the lead connector elements can be intermittently or continuously disrupted. Connection mechanisms, such as set screws, may be insufficiently tightened at the time of implantation, followed by a gradual loosening of the connection. Also, lead pins may not be completely inserted. In some cases, changes in leads or connections may result in intermittent or continuous changes in lead impedance.
Short circuits, open circuits or significant changes in impedance may be referred to, in general, as lead related conditions. In the case of cardiac leads, sensing of an intrinsic heart rhythm through a lead can be altered by lead related conditions. Structural modifications to leads, conductors or electrodes may alter sensing integrity. Furthermore, impedance changes in the stimulation path due to lead related conditions may affect sensing and stimulation integrity for pacing, cardioversion, or defibrillation. In addition to lead related conditions, conditions associated with sensor devices or sensing circuitry, as well as conditions associated with electrodes or sensors not located on leads, may affect sensing integrity.